1. Technical Field
The present invention generally relates to devices and methods for assessing a temperature exposure history of a sample, e.g., the maximum temperature that the sample has attained since the last time it was prepared for use. In particular, the invention relates to such devices and methods that assess the temperature experienced by a nonmetallic sample comprising a nonmetallic material of an optionally disordered and/or multicrystalline microstructure by detecting for changes in the electronic state of the material.
2. Related Art
Various products and components may be damaged when exposed to temperatures above a critical level. For example, in the course of normal operation, turbine blades may be exposed to a wide range of temperatures. Turbine blades may be damaged when exposed to high peak temperatures, but the damage caused by excessive heat may not be readily apparent from casual observation. As turbine blades are often put to use at high speeds, the consequences associated with the blades' failure in operation may be catastrophic. Accordingly, there is a need to determine the temperatures to which such items have been exposed so as to determine whether such items should be replaced. In particular, warrantors and manufacturers of such items are particularly interested in temperature exposure histories to determine whether the items have been exposed to temperatures outside the intended operational temperatures.
Various temperature sensor technologies have been developed to provide an indication of the temperature history of various products and components. For example, indicators have been developed to assess temperatures in experimental internal combustion engines, where conventional methods of temperature measurement were impractical. The indicators employ metals and alloys, which undergo a permanent change in hardness when they are subjected to high temperatures. However, there are a number of disadvantages associated with taking hardness measurements on the indicators to measure the temperature exposure history of the products and parts. For example, hardness measurement are considered destructive or semi-destructive, because once a measurement has been made it cannot be repeated over exactly the same spot. In addition, hardness measurement typically must be done under controlled laboratory conditions.
U.S. Pat. No. 5,735,607 to Shahinpoor et al. describes shape memory alloys having properties that can be useful for use in temperature history sensors. Such alloys can be trained to have a certain shape in its austenitic state or at temperatures above the alloy's austenitic finish temperature. The alloys move in a certain fashion to a second shape, its martensitic state, which is a softer state for the material, when the temperature drops below the austenitic finish temperature and eventually reaches below the martensitic start temperature. The alloys will not return to the martensitic shape without additional external force even if the temperature subsequently falls below the austenitic temperature.
Other known temperature sensors do not generally provide a persistent record of temporary temperature deviations. While conventional temperature sensors such as thermometers or thermocouples provide a continuous indication of the temperature of the material at any particular instant in time, they do not provide a permanent indication of out-of-range temperatures. Instead, such sensors require an additional permanent recording apparatus.
To provide a permanent indication of the temperature history to which an item has been exposed in operation, a temperature sensor may be used to generate electronic temperature signals to a microprocessor/microcontroller (e.g., via an analog-digital converter). In turn, the microcontroller may be used to convert the signals to temperature readings and to store peak temperature data in nonvolatile memory. As described in U.S. Pat. No. 6,377,903 to Weber, such techniques may be applied to a steel rolling mill. Similarly, U.S. Pat. No. 5,025,267 Schofield et al. describes a similar technique for determining the peak temperature of a thermal print head.
One problem with such techniques is that the microcontroller may not be able to withstand the same temperatures as the sensor and must be isolated from the environment whose temperature is being measured by the sensor. That is, the microcontroller may have to be placed in an environment that is, at all times, friendly to electronics associated with the microcontroller. For example, jet engine turbine blades are typically operated at temperatures well in excess of the −40° C. to 85° C. temperature range for proper operation of microcontrollers and associated electronics. Another problem associated with such techniques is that the microcontrollers and accompanying electronics must be continuously powered when in use. Another problem, common to thermocouples, thermistors, and resistance temperature detectors, is that they must be connected to the microcontroller, or to an accessory device, by extension wires which complicate the installation and pose durability problems
Optical temperature sensing methods have been developed as well. For example U.S. Pat. No. 4,515,474 to Fox describes optical-fiber systems and methods for determining the most extreme temperature prevailing along the length of an optical-fiber light guide. However, this method suffers from drawbacks similar to those associated with the above-described microcontroller methods.
X-ray diffraction techniques have also been suggested as having utility for measuring the maximum temperature experienced by a single crystal silicon carbide sensor. For example, Volinsky et al. (2004), “Irradiated cubic single crystal SiC as a high temperature sensor,” Mat. Res. Soc. Symp. Proc. 792:R5.3.1-R5.3.6, describes single crystal SiC sensors that may be incorporated into gas turbine blades, space shuttle ceramic tiles, automobile engines, etc. The maximum temperature to which the sensor is exposed may be determined by using X-ray diffraction to measure whether radiation-induced strain via neutron bombardment is relieved via annealing and/or whether annealing effects volumetric change.
Luminescence readers have been used to analyze materials that are crystalline and able to store part of the energy imparted to the material by interaction with ionizing radiation. The interaction of ionizing radiation with a nonmetallic material may result in charge-carrier redistribution within the material. Some part of the redistributed charge carrier population may become trapped at defects in the material. The trapped charge carrier may then be released when the material is stimulated via optical or thermal energy. A fraction of the released charge carriers will recombine, which results in luminescence. The intensity of the emitted luminescence is related to the amount of trapped charge carriers that was released by the stimulation. Such a mechanism has been used in “radiation detector badges” worn by personnel in hazardous locations. If the badge has been exposed to ionizing radiation, some part of the charge carrier population will have become trapped. Release of these carriers by thermal or optical interrogation yields a signal that can be interpreted as a measure of the total amount of ionizing radiation to which the badge has been exposed.
Thermoluminescence readers have found use in dating archaeological and geological materials. For example, thermoluminescence dating has been used to date buried objects that have been heated in the past (e.g., pottery). Immediately after heating, the thermoluminescent signature of the object is assumed to be “reset.” Since the dose of ionizing radiation accumulated by the object from radioactive elements in the soil, cosmic rays, etc., is proportional to age, the age of the objects may be estimated by dividing the accumulated dose of received radiation received by the assumed dose accumulated per year.
Thermoluminescence technology has also been used in dosimetry applications. In such a case, powders having empty traps are used as a measure of exposure to ionizing radiation. X-ray technicians may use such technologies in the form of exposure badges that incorporate such powders as a safety measure to reduce the chances of being to exposed radiation without the technician's knowledge.
To overcome the drawbacks associated with known technologies for assessing the temperature exposure history of a sample such as, for example, a turbine blade or vane, or combustor liner, or a computer chip or any other piece of equipment, new methods have been discovered that use trapped charges to infer thermal information. In particular, it has been discovered that luminescent technology may be used to assess the temperature exposure history of a sample of a nonmetallic material, e.g., to determine the peak temperature to which the sample has been exposed.